Splitting them out allows for plugins to use _test/phpcs.xml as the
basis for their own linting without having an overly permissive coding
standard.
Also, this makes it more obvious and painful that these are just
intended as temporary exceptions and should be actually fixed.
The rule `Generic.ControlStructures.InlineControlStructure.NotAllowed`
has its comment adjust to make it clear that this is an intended
deviation from the PSR-2/PSR-12 coding standard.
The rule `PSR1.Classes.ClassDeclaration.MissingNamespace` has to remain
in the DokuWiki coding standard as the plugin base classes can currently
not reasonably be in namespaces.
Andreas Gohr